When Worlds Collide
by Water Mage Chick96
Summary: AU. In this world, there is good and evil. Unfortunately, the good is normally the ignorant. The evil mostly consists of blood thirsty demons that prey upon the innocent. It is the duty of demon slayers to protect the innocent. What happens when a certain blonde slayer causes a huge impact in the current war? Is it for the better? For the worse? You'll have to find out! FxM
1. Chapter 1

**When Worlds Collide**

**Author's Note:**

** Hey! I am a terrible person! I have like four, possibly five other stories to update! And what do I do? What do I do?! I let my imagination run wild and come up with ideas! Yes, ideas! So here I am, with yet another Fanfic! I hope you enjoy this and bear with me people! . I will keep up with my other stories, and quite honestly, I put "Fighting Against a Deteriorating World" above my other stories, so any readers out there who follow that specific fanfic, needn't worry!**

** On another note, I am writing another one-shot and it is a Fiolee of course. ****_Just like all of my Adventure Time Fanfics. _****If I get enough people telling me to continue that one-shot, then I'll make it a multi-chapter story-because really, it could go either way! So, I will let you guys know in one of my other author's notes when I finally have that posted. Also, I am going to try and be as consistent as possible with all of my updates, however I do have but loads of work to do. Mainly my art assignments-so it's pretty important. I apologize for my updates being slower, but I'm pretty swamped this month!**

**Anyway, I'm also posting up the second chapter to this at the same time so to have a decent start off :P And "Fighting Against a Deteriorating World," I will update within the week-along with "Tears to Shed" which I meant to update sooner! I of course, got distracted with other stuff and then I got this plot bunny in my head that I ****_had _****to write out!**

**Summary:**

** In this world, there is good and evil. Unfortunately, the good is normally the ignorant. The evil mostly consists of blood thirsty demons that prey upon the innocent. It is the duty of demon slayers to protect the innocent. What happens when a certain blonde slayer causes a huge impact in the current war? Is it for the better? For the worse?...I really suck at summaries, just read to find out :P**

**Also another note just a heads up, Fionna will be OOC for the beginning portion of this story, not for too long though. But there is reasoning behind it. I just thought I'd let you guys know because the reasoning for her OOCness won't be explained until this story is a few chapters in and even then there will be more to it. There will be hints in the meantime but…its part of the plot, so just read and find out .**

**Chapter One: A Change in Pace**

**Disclaimer: I do not own Adventure Time**

She walks down the winding roads, in an eerily calm demeanor. Golden locks cascading down her back and shoulders, gently swaying along with the light autumn breeze, her heavy worn out dark boots-which contrasts heavily to her innocent features-echo with a light _thud, _her faded light blue jeans are stretched around the knees and frayed at the edges-a sure indication of use_; _her stance and stride shows the prime example of perfect posture-which evidently contributes to the way she seems to walk like a perfect soldier. She straightens her large green pack as the blue pools of her irises roam, weaving through her surroundings in calculating manner_. _Any passing by human traffic keeps their distance, as if the girl constantly radiates an invisible shield, protecting her from the outside world. _If only._

The young blonde comes to a sudden halt. She takes in the sight of an old apartment building that rests before her eyes. The building looks like a house that was built about forty years ago; the structure looks as if it's on the verge of collapse, _just what I need._ Rather than the dual apartment being segregated horizontally, the structure is instead separated vertically. The girl drops the large green pack with a loud _thud_, relieving the minor weight that's been burdening her shoulder. Heaving out a sigh, the teenager drags her pack dejectedly behind her heels as she reluctantly approaches the old chipped away building that she soon has to adjust to calling _home. _

**_Weeks Prior…_**

**_ "_**_Demon Slayer Fionna Mertens." A short pause resonates throughout the room, indicating the man to continue on with his speech. _

_ "You are to be relocated within the next few weeks. It is urgent that one of my star slayers watches over this town." The blonde hesitantly, yet steadily responds,_

_ "What about here master? Isn't it important that I stay here?" The older man in question releases an anguished sigh._

_ "Yes, but your new location is in dire need. There has been an escalating rate of murders. Demons are overrunning this town. We need to send the best to restore balance once more. Any more questions?" Silence momentarily sweeps over the room,_

_ "Where am I going?"_

_ "Well, as the locals say. You are going to "A.A.A"_

**_~Present~_**

Fionna approaches her knew front door. The dark oak entryway relents with a groan, excess chipped paint that had been sloppily splayed across the wood as an act of touch-up falling while doing so. _Home sweet home. _

The old apartment is decently spacious. Right away she is led into a short hallway, off to the left lays a rather large kitchen with appliances possibly ranging back to the nineteen seventies, directly across the entryway door was a somewhat tightly spaced bathroom, and off to the right the short hallway leads into a living room, in between there are two doors-she assumes her bedroom and possibly a linen closet. After further speculation, the blonde notes that the living space is provided with the bare minimum of furniture. With a shrug, she hauls her pack once again slinging it round her shoulder somewhat properly once more. _Good enough for now._

* * *

After a short lived exploration and unpacking duty, Fionna plops herself on the lumpy mattress that she is going to have to adjust to over the period of time that she has to live in this town-_however long that will be_. Trying to be productive with herself, she reels in her thought,

_I am the only slayer in town. I'm going to have to blend in and transfer to the local high school starting tomorrow. Every night I have to patrol the area. I have to protect the innocent….I have to protect…even if I die while doing so._

She stares at the ceiling, expressionless eyes flicker from ceiling tile to ceiling tile as if to solve the intricate pattern, her facial features simply splay in boredom, she feels no emotion, her thoughts aren't buzzing with loads of questions-the blonde is calm and collected as per usual, accepting whatever has to be done in order to carry out the duty of protecting innocent lives. _No matter the cost…_

**Author's Note:**

** Sorry the first chapter was short and poorly written! For whatever reason, I always suck at writing introductions to my stories. I feel like this chapter is a turn off….:( But no worries! My writing will definitely pick up in the next chapter now that I've got the obstacle of the first chapter over with! ^.^ Please review! I'd really love to know what you guys think, whether it'd be critiques, ideas, advice, ect, ect. Ta ta for now!~**

**-J.M**


	2. Chapter 2

**Quick Author's Note:**

** If you haven't noticed, the ****_italicized _****words indicate thought. I know in my other stories it's usually minor thoughts and emphasized words, however this story is written in third person so i thought it necessary. Just thought I'd mention this in case of confusion :)**

** And also, Marshall Lee isn't going to be a demon in this story! I just thought it'd be a little ****_too_**** cliché. I cannot even tell you how many different stories I've read where the protagonist of a plot falls for someone who seems to be on the enemies side! Though, I'm sure many of you have read a bunch of these, whether they were fanfictions or not :/**

**…I guess I prefer to be unpredictable! XD**

**Chapter Two: New School, First Impressions**

**Disclaimer: I do not own Adventure Time**

5:30 a.m. One hour before the slayer must venture to the world of _high school._ More specifically, "The Academy of Advanced Arts," where all of the large town's gifted youth attends. Rather than prepping for the day ahead, the blonde continues to viciously toss and turn in a restless slumber.

Sheets scatter about the room, cold sweat trickles down the young woman's temple, limbs thrash around reaching out for some source of comfort, but none is found. Rather than any dream or nightmare, horrid memories continue to echo and haunt the girl's mind-devouring any sanity she had managed to grasp over time. Bits and pieces of shattered themes reiterating within the dark confines of her suppressed memory, the constant echo shakes her entire being-

_"She will be the strongest"_ Trying to fight off the plaguing dream, her hands fold into fists, gripping until her knuckles transition to a searing shade of white,

_"She will kill!"_ Fionna's stiff body jolts to life, tearing her away from the relentless dream; a sigh escapes her peachy plump lips. Huffing, she inhales deeply then follows through by releasing the deep intake. _Will the dreams ever end?_

Blue irises dance across the room and rest on an old cracked mirror from across her bed. Fionna shakily lifts herself up and strides to the bronze rimmed mirror with a slight stagger. Straightening herself, she clamps each hand on either side of the frame against the sturdy wall. Her attention hesitantly averts to that of the glass, peering into her own reflection. Sure enough, her eyes are that of a very dark inky blue-she barley distinguishes where her pupil begins and ends. Her grip painfully tightens. _I won't let it possess me!_

Screwing her eyes shut, she stamps out the excess tormenting thoughts that have her on hay wire. _Breathe in._ She sucks in a breath. _Breathe out. _The exhales bounces off the dull gray walls. _ I am in control!  
_Shooting up her head to the cracked and dusty mirror once more, mouth set in a firm line, brows knitted in determination, Fionna opens her eyes once more, relieved to see what displays in front of her. No longer the inkiness of blue that practically consumes her pupil, no longer the bright and joyful sapphire that the girl had come accustomed to as a young child, but the border line hazy, lifeless midnight blue stare that seems to have become the balance of the two. Fionna drops her arms to her sides; her entire being loosens, alleviating the previous fright. Turning round, her lax gaze darts to the bright blue digital clock; _time for school._

* * *

Large crisp white buildings dominates the lot, students idly walk to and fro the different scattered buildings. Many 'popular' students stride with a slight skip indicating that they 'own' the school, some walk merrily by with not but a care in the world, some strut in an intimidating matter asking to be messed with, others dart their attention with high alert with their persona dripping in fear. The young slayer continues to observe in that calculating matter yet again, she stiffly follows the bright brick pathway leading into the school's gates. _Typical_ _high school life._

Clad in the school's dark blue and gray uniform, Fionna makes her way through the iron passage way when not but feet away she hears,

"_Alright,_ fresh _meat! _I'm the boss around here! Step out of line and you'll have the pleasure of meeting my fist! Do as I say and _maybe_ your life won't be a living hell for the next year! Any questions?!" A shrill voice laced with venom that of teenaged girl shouts. The slayer's stride halts,

"Just because you're a senior doesn't mean-_ouch!" _The slightly premature voice of a girl gets cut off as the senior in question viciously pulls the brunette down against the asphalt by her hair. Fionna shifts her attention to the scene beside her, her glare shone in that of a different expression, shocking bright sapphire reaching the surface as a hell fire ignites within her being. _Here comes Fionna, the heroine,_

"Hey _bitch!" _The white haired senior or said 'bitch' meets Fionna's frosty glare.

"What'd you call me just now?!" Unfazed Fionna marches up to the bully with a stone fiery glare.

"You think you can just act like you're better than these kids just because, for whatever reason, you think you own the place?! Let me tell you, you're just some snobby tyrant with no bark to her bite who goes and preys on people who you _know_ are weaker! Who won't have the nerve to stand up to you!

"People like you really irritate me! And I'm not going suppress myself from throwing up the entire year listening to your 'reign of the school!" The scarily thin senior in response, releases the hair of the brunette, stepping up in suite,

"Listen up honey, my name is Ashley. _Remember that._ Step out of line again, and I'd be happy to show you your place. This is my territory and I'm not going to be shown up by some blonde haired bimbo new girl. You might think you're tough by standing up to me and calling me out, but you're _not_ and you _never_ will be. _Got it?_" The last of Ashley's response comes out more of a growl and is barely coherent. The other girls visibly shake as what seems to be the rest of the school's student body circles round the scene, straining to hear every last word spilling out of the blonde and the 'queen' of the school.

Straightening herself even further, Fionna doesn't back off, she proceeds to step up-which evidently, emphasizes the major height difference of the tall white haired bully and the average height Fionna.

"Either leave these girls alone, or _fight me._ That is, unless you're scared" The blonde quirks an eye brow mockingly as the 'queen' flushes in anger, Ashley steps back and fists a pale manicured hand readying to punch her new found enemy. Sloppily, she reels her slightly scrawny arm back with all her might and releases her blunt attack to the blonde-

"Is that all you've got?" Ashley's hand makes a loud _slapping_ sound as it collides into Fionna's firm palm. The surrounding students resound in murmurs of astonishment as the new girl _catches_ Ashley's punch. No pain shown what so ever. In a blind furry, the bully struggles as she attempts to land a hard kick to the blonde, but instead, her face is met with the hard ground.

"For someone who likes to pick fights, you sure _can't_ fight" Fionna exclaims mockingly as she roughly pins Ashley to the hard ground.

All the students stare in shock, jaws slack, and eyes wide. In the midst of all of this, a certain raven haired boy reacts in suit as his so called 'friends' nudge him,

"_Your girlfriend just got her ass handed to her!" _After the chilling pause, a clap resounds, followed by more claps, followed by cheers, and soon, the whole student body celebrates joyfully towards the recently played out event. A small smirk grows on the boy's features.

Fionna stills in response to the cheering. _How long had they been standing there?_ She releases her iron grip and gets up rather stiffly; her eyes become half lidded as the adventuring shock of sapphire dissipates once more into a midnight haze. Her shoulders slack down as if to rid the tension as she calmly vacates the scene, not caring that all eyes are currently boring into her in a confused manner. No one dares to follow.

**Author's Note:**

**Okay, so I cut the chapter there. What do you guys think so far? Of course I'm trying to take this, much like my other plots, slowly. Anyways, I hope you guys are enjoying this so far! Please review and let me know! Critiques, thoughts, suggestions, etc.-I accept and would like to hear them all! :D Ta ta for now!~**

**-J.M**


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